Avoid prematurely unlinking streams in send_reset, in some cases. (#319)

Because `send_reset` called `recv_err`, which calls `reclaim_all_capacity`,
which eventually calls `transition(stream, ..)` -- all of which happens _before_
the RESET frame is enqueued -- it was possible for the stream to get unlinked
from the store (if there was any connection-level capacity to reassign). This
could then cause the stream to get "leaked" on drop/EOF since it would no longer
be iterated.

Fix this by delaying the call to `reclaim_all_capacity` _after_ enqueueing the
RESET frame.

A test demonstrating the issue is included.

tests/h2-tests/tests/client_request.rs

@@ -1061,6 +1061,79 @@ fn drop_pending_open() {
     client.join(srv).wait().unwrap();
 }
 
+#[test]
+fn malformed_response_headers_dont_unlink_stream() {
+    // This test checks that receiving malformed headers frame on a stream with
+    // no remaining references correctly resets the stream, without prematurely
+    // unlinking it.
+    let _ = ::env_logger::try_init();
+
+    let (io, srv) = mock::new();
+    let (drop_tx, drop_rx) = futures::sync::oneshot::channel();
+    let (queued_tx, queued_rx) = futures::sync::oneshot::channel();
+
+    let srv = srv
+        .assert_client_handshake()
+        .unwrap()
+        .recv_settings()
+        .recv_frame(frames::headers(1).request("GET", "http://example.com/"))
+        .recv_frame(frames::headers(3).request("GET", "http://example.com/"))
+        .recv_frame(frames::headers(5).request("GET", "http://example.com/"))
+        .map(move |h| {
+            drop_tx.send(()).unwrap();
+            h
+        })
+        .wait_for(queued_rx)
+        .send_bytes(&[
+            // 2 byte frame
+            0, 0, 2,
+            // type: HEADERS
+            1,
+            // flags: END_STREAM | END_HEADERS
+            5,
+            // stream identifier: 3
+            0, 0, 0, 3,
+            // data - invalid (pseudo not at end of block)
+            144, 135
+            // Per the spec, this frame should cause a stream error of type
+            // PROTOCOL_ERROR.
+        ])
+        .close()
+        ;
+
+    fn request() -> Request<()> {
+        Request::builder()
+            .uri("http://example.com/")
+            .body(())
+            .unwrap()
+    }
+
+    let client = client::Builder::new()
+        .handshake::<_, Bytes>(io)
+        .expect("handshake")
+        .and_then(move |(mut client, conn)| {
+            let (_req1, mut send1) = client.send_request(
+                request(), false).unwrap();
+            // Use up most of the connection window.
+            send1.send_data(vec![0; 65534].into(), true).unwrap();
+            let (req2, mut send2) = client.send_request(
+                request(), false).unwrap();
+            let (req3, mut send3) = client.send_request(
+                request(), false).unwrap();
+            conn.expect("h2").join(drop_rx.then(move |_| {
+                // Use up the remainder of the connection window.
+                send2.send_data(vec![0; 2].into(), true).unwrap();
+                // Queue up for more connection window.
+                send3.send_data(vec![0; 1].into(), true).unwrap();
+                queued_tx.send(()).unwrap();
+                Ok((req2, req3))
+            }))
+        });
+
+
+    client.join(srv).wait().unwrap();
+}
+
 const SETTINGS: &'static [u8] = &[0, 0, 0, 4, 0, 0, 0, 0, 0];
 const SETTINGS_ACK: &'static [u8] = &[0, 0, 0, 4, 1, 0, 0, 0, 0];